Lightning-induced high temperature and pressure microstructures in surface and subsurface fulgurites

Li-Wei Kuo,Ching-Shun Ku,Dennis Brown,Wen-Jeng Huang,Marianne Negrini,Steven A F Smith,Tze-Yuan Chen,Chien-Chih Chen,Ching-Yu Chiang

doi:10.1038/s41598-021-01559-x

Abstract

Cloud-to-ground lightning causes both high-temperature and high-pressure metamorphism of rocks, forming rock fulgurite. We demonstrate that a range of microstructural features indicative of high temperatures and pressures can form in fulgurites at the surface and in fractures up to several meters below the surface. In comparison to a granite reference sample collected from a borehole at a depth of 138 m, microstructures in both the surface and fracture fulgurite are characterized by: (i) the presence of glass, (ii) a phase transformation in K-feldspar with the presence of exsolution lamellae of plagioclase, and (iii) high residual stresses up to 1.5 GPa. Since this is the first time that fracture-related fulgurite has been described, we also carried out a 1-D numerical model to investigate the processes by which these can form. The model shows that the electric current density in fractures up to 40 m from the landing point can be as high as that on the surface, providing an explanation for the occurrence of fracture-related fulgurites. Our work broadens the near-surface environments in which rock fulgurite has been reported, and provides a detailed description of microstructures that can be compared to those formed during other types of extreme metamorphic events.

Highlights

Cloud-to-ground lightning causes both high-temperature and high-pressure metamorphism of rocks, forming rock fulgurite
Glass is commonly interpreted as a high-temperature product formed by melting and has previously been reported in rock fulgurites[8], tectonic faults[18], hypervelocity impacts[19], and along the base of large landslides[20]
While glass is a common component of rock fulgurite, our data provide evidence for the presence of high-temperature and high-pressure features in both K-feldspar and plagioclase; namely plagioclase exsolution in K-feldspar, the change from a monoclinic to triclinic crystal structure in K-feldspar, a decrease in twin density in plagioclase as compared to the reference sample

Summary

Introduction

Cloud-to-ground lightning causes both high-temperature and high-pressure metamorphism of rocks, forming rock fulgurite. In the vicinity of the landing point, cloud-to-ground lightning can cause shockwave pressures in excess of 10 GPa and temperatures above 1700 °C, resulting in high-temperature and high-pressure features being formed in the target material, which is sometimes converted into a sand or rock fulgurite (e.g.,3–9). We document in detail the composition and microstructural features of rock fulgurites that were formed by recent cloud-to-ground lightning events on Kinmen Island, Taiwan. This allows us to recognize and report the first known occurrence of rock fulgurite formed within fractures extending from the surface to a depth of several meters, suggesting that the extreme metamorphic effects of lightning are not restricted to the present-day topographic surface. Some of the microstructural and petrographic characteristics of the fulgurite samples described here are reminiscent of those reported to form during low-level shock metamorphism, and our observations have consequences for the interpretation and distinction of extreme metamorphic events

Methods

Results

Conclusion